1. Field of the Invention
The present invention relates generally to inductive amplifiers used to identify and trace a particular wire within a group of wires, and more particularly, to an inductive amplifier having a filter for suppressing unwanted noise signals, including noise induced by nearby power lines.
2. Description of the Relevant Art
Inductive amplifiers, or tone detectors, are high impedance amplifiers used by telecommunications service personnel to identify and trace a particular pair of wires within a group or bundle of wires, without damaging the insulation surrounding each of such wires. Such inductive amplifiers are commonly used in conjunction with tone generators for tracing wires. The tone generator is coupled with a particular pair of wires of interest to inject a distinctive audio frequency tone within such wires. Typical audio tone generators are capable of providing two different tones within two different frequency ranges. A first, or lower, tone is provided within the frequency range of 500 Hz. to 1100 Hz. A second, or higher, tone is provided within the frequency range of 800 Hz. to 1600 Hz. The output of the tone generator can provide one of such tones continuously, or can be caused to rapidly switch between the lower and higher tones to produce a warbled output tone. Accordingly, most inductive amplifiers are designed to be responsive to frequencies within the range of approximately 300 Hz. to 3,000 Hz.
During use, the probe tip of the inductive amplifier is placed adjacent one of several wires to identify the particular pair of wires conducting the injected audio tone. The inductive amplifier typically includes a built-in loudspeaker which can be used to produce an audible signal derived from the injected audio tone. In addition, terminals may be provided for allowing a rugged service telephone handset, or buttset, to be attached to the inductive amplifier to reproduce the detected audible tone in the earphone of the buttset.
While the above-described method of tracing wires ordinarily works well, noisy environments can pose problems that interfere with the normal use of such inductive amplifiers. In a noisy environment, a power line "hum" may be present, either directly from power lines or from appliances such as fluorescent light bulbs, radios, air conditioning equipment, etc. Electrical power within the United States is typically provided as an alternating current signal having a frequency of 60 Hz., and such power line "hum" is composed of a 60 Hz. signal plus harmonics thereof. Noise levels are particularly high in areas where power distribution panels for a building are located. Often, the amplitude of such "noise" is much greater than the amplitude of the signal output by the tone generator; in this case, the power line "hum" masks the desired signal tone output by the tone generator, thus making the location of the desired pair of wire difficult. In such instances, all that a service technician can do is to directly connect the inductive amplifier to the wire under test, as by stripping away insulation from the wire; however, this practice may not be either feasible or desirable.
Those skilled in the art are familiar with so-called band-pass filters wherein only signals within a particular band of frequencies are permitted to be passed through an amplifier. In the case of inductive amplifiers, the use of such a band-pass filter permitting only signals in the frequency range of approximately 300 Hz. to 3,000 Hz. may assist in the rejection of 60 Hz. "hum". However, noise signals often include harmonics of the fundamental noise frequency. Thus, power line "hum" typically includes not only 60 Hz. frequency signals, but also noise signals having harmonic frequencies of 120 Hz., 240 Hz., 480 Hz., 540 Hz., 600 Hz., etc. Accordingly, while a band-pass filter might suppress the fundamental 60 Hz. frequency, a band-pass filter can not be used to fully-suppress higher-frequency harmonics that overlap with the desired frequency range of the audio tone supplied by the tone generator.
Accordingly, it is an object of the present invention to provide an inductive amplifier adapted to amplify the tone signals provided by most conventional tone generators, while suppressing power line "hum" and other noise signals present in a noisy environment.
It is another object of the present invention to provide such an inductive amplifier which rejects not only the fundamental power line frequency, but which also rejects harmonics of such fundamental frequency.
It is still another object of the present invention to provide such an inductive amplifier which does not interfere with the passage and amplification of the tones output by the tone generator.
A further object of the present invention is to provide such an inductive amplifier which can be manufactured easily and inexpensively.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.